Rotary fuel pumps driven by an electric motor have been utilized for some years in some vehicles either as original equipment or as appliances to supplement the original fuel supply system. The pump and power unit are frequently in a common housing as shown, for example, in U.S. Pat. No. 4,401,416, issued Aug. 30, 1982 to Charles H. Tuckey.
Since the pumps are frequently mounted in the fuel tanks of a vehicle, the noise factor is extremely important. A pump under load will normally produce more noise and this may be audible as a humming noise, to an annoying degree, to passengers in the vehicle.
It will be appreciated that in the pumping cycle, as one pumping cell is exhausting, another cell is taking in fluid at the same time. In other words, intake and exhaust pressure waves are timed with one another, and normally the quantity of fluid being exhausted from each cell is the same as that being taken in by another cell. It has been noted that pressure waves or pulses are present at the inlet, as well as the outlet, at all operating pressures.
It is an inherent characteristic of a positive displacement pump to produce slight pressure pulses each time one of the multiple vanes passes through its pumping cycle. For example, a roller vane rotary pump produces an audible humming noise when operating at system pressure. This noise has a tendency to increase as the output pressure requirement is increased.
One must acknowledge and deal with the extreme pressure differential between the inlet and exhaust sides of the pump. For instance, the inlet zone is usually at an average pressure close to atmospheric; and the outlet zone average pressure is much higher, i.e., 60 psig or more depending upon the operating pressure requirement of the pump.
It has been a desire of manufacturers and users of positive displacement rotary pumps to reduce or eliminate pressure pulses in order to achieve a smooth, pulse-free flow of fluid out of a pump at desired operating pressure.
Hollow pulse absorbing chambers in fuel pumps have been proposed previously as exemplified in U.S. Pats. to Yoshifumi, No. 4,181,473, issued Jan. 1, 1980 and to Tuckey, No. 4,521,164, issued Jun. 4, 1985. U.S. Pat. No. 5,035,588 issued Jul. 30, 1991 to Charles H. Tuckey discloses a hollow pulse modulator of a flexible plastic material formed by a blow molding process which has air trapped therein.